Since the work done by C. DIDCOTT on the dilation and the support of anatomical conduits, the concept of dilatable endoprostheses has enjoyed great success.
One of the most remarkable breakthroughs in this field concerns in particular cardiovascular surgery, especially the reduction of aneurysms and the opening of stenoses.
A consequence of the general rate of success of these methods has been increasing demands on the part of practitioners, both as regards the quality of the products brought onto the market, and also their ease of use.
Crucial criteria in this regard include the high ratio between the diameter of the endoprosthesis in its contracted shape and its nominal diameter (unconstrained state), but also the flexibility of this endoprosthesis, which must be able, during insertion, to follow sinuous courses without thereby causing kinking.
Furthermore, when it is in place, such an endoprosthesis must demonstrate mechanical characteristics compatible with those of the vessels being treated, and it must be able to withstand the crush stresses generated by the ambient pressure and by the presence of adjacent organs.
Research initially focused in particular on blood vessels of small and medium calibre, but much remains to be done in the field of vessels of very small diameter and, by contrast, in anatomical conduits of large diameter.
Treating thoracic and abdominal aneurysms thus requires the use of endoprostheses of large diameter: of the order of 35 to 45 mm for thoracic aneurysms, and of the order of 22 to 33 mm for abdominal aneurysms.
None of the endoprostheses available on the market in this diameter range entirely satisfies the expectations of practitioners at the present time, essentially because they are unable to fulfil their role long-term, they are not easy to use, or else because the materials used are not suitable.
The endoprostheses used to repair the anatomical conduits comprise a rigid framework which is often provided with a coating. The endoprostheses consisting is solely of a framework bear the name "stent".
There are basically two types of frameworks (or stents) on the market, namely frameworks which are dilated by inflatable balloons, and self-expanding frameworks which comprise braided or unbraided structures.
Endoprostheses are known which are put in place and then dilated to their nominal diameter by introduction of an inflatable balloon.
Particular disadvantages of this technique are the interruption in the blood flow and the dimensions of the framework.
The balloon stents can only be used for treating lesions in arteries of small calibre (at most 12 mm). The reason for this is simple: for a stent with, for example, an initial diameter of 3 mm to be dilated up to a diameter of 8, 10 or even 12 mm, it is necessary to use a pressure of up to 5 to 10 atmospheres (as indicated in U.S. Pat. No. 4,950,227).
The balloon must therefore be extremely strong, which entails problems as regards diameter.
Furthermore, it is not possible to treat long lesions using this technique.
It should be noted that an intervention performed on an abdominal aneurysm can last for 6 to 8 hours when using a femoral or iliac surgical approach (compared to an average duration of 2 hours for treatment by a direct surgical route).
As regards the self-expanding stents, these do not require balloons: they are generally stretched out lengthwise and introduced, in a shape with a reduced diameter, into an applicator consisting of a tubular catheter equipped with a pusher. The whole assembly is introduced, particularly by the femoral or iliac route, as far as the deployment site, where the endoprosthesis is released.
Although they have some advantages, the known models of self-expanding stents also have a number of limitations, long regarded as insurmountable. Their diameter does not generally exceed 25 mm.
The braided stents with cobalt/nickel/chromium alloys (ELGILOY.RTM. or PHYNOX.RTM.), however, permit diameters varying from 2 mm to 45 mm or even 50 mm to be obtained.
Upon release, the endoprosthesis, initially subjected to elongation, with narrowing of its diameter, automatically recovers its nominal diameter.
The first braided endoprostheses of this type were made by C. DIDCOTT.
FR-1,602,513 discloses endoprostheses provided with a rigid framework which is formed by interweaving metal filaments into a braid. This document describes braids having an angle of intersection .sub..alpha. of between 45 and 90.degree. between the filaments of two different layers.
It goes without saying that, strictly from the mechanical point of view, a braid resists crushing less effectively, the more the braided filaments from which it is built deviate from a quasi-annular structure, namely a spiral of very small pitch, corresponding to an angle as close as possible to 90.degree. relative to the axis of the braid angle (meaning that angle .sub..alpha. between filaments should be as close as possible to 180.degree. C. i.e. actually around 120.degree. C.) (as described in FR-2,333,487). The smaller this angle, the less effectively the braid resists crushing.
Patent U.S. Pat. No. 5,061,275 describes an endoprosthesis with a braided framework in which the angle of intersection .sub..alpha. is obtuse. In this case, the coefficient of elongation of the prosthesis is high, which entails problems when it is being put into place. (Coefficient of elongation is defined as the ratio of the axial extension of such a prosthesis in its stressed shape, hence with reduced diameter, and in its unstressed shape, at its nominal diameter).
Releasing this type of endoprosthesis therefore requires long practice, as pinpointing it is difficult (the endoprosthesis undergoes considerable shortening at the moment of its release). The endoprosthesis takes up a substantial length in the introducer, which creates friction and reduces manoeuvrability.
Research workers who have set themselves the task of solving the problems associated with the use of self-expanding prostheses with mechanical action have come up against questions relating to the angle, thickness and composition of the filaments, without managing to obtain a prosthesis bringing together all the quality criteria: it has not been possible to obtain a prosthesis combining a low angle of intersection and good resistance to crushing.
It will also be noted that for a same angle .sub..alpha.= 85.degree., a braid with 32 filaments presents a resistance to radial pressure which is 50%) higher than a braid with 24 filaments of identical diameter, a fact which shows that such a structure responds to relatively complex relationships.
EP-A-0 740 928 describes a braided endoprosthesis made of cobalt/nickel/chromium-based alloy, in which, in order to increase the resistance to radial compression, a doubled filament has been used, which poses a problem as regards the space taken up in the applicator.
The use of such filaments for making medical braids should in principle give good results. However, the limit of resistance to rupturing of the cold-hammered filament is situated at about 2000 N/mm.sup.2 and, after thermal treatment, the filament reaches values of resistance to rupture of 2500 to 2700 N/mm.sup.2, which makes the filament rigid and brittle; they prove relatively difficult to wind up and braid on account of their inherent elasticity. Frequent breaking of the filaments spoils in particular the spindles of the machines, which are subjected to accelerated deterioration.
In addition, when used long-term, especially for vascular conditions where the stresses on the metal are very high (e.g. abdominal aneurysms), it was found that the stents made from these filaments aged rapidly (effects of fatigue).
Fatigue tests have shown the same results after simulated longitudinal compression for an equivalent period of five months.
Other self-expanding endoprostheses described for instance in U.S. Pat. No. 5,354,309 and U.S. Pat. No. 5,540,713 are characterized by memory alloy part having a cylindrical jacket-shaped outer contour. For instance nickel/titanium alloys such as Nitinol.RTM. may be used.
Different shapes are known: truncated incised cylinders, helical structures, mesh structures, rolled-up metal sheets and the like.
When they reach the body temperature, they tend to adopt a radially expanded shape which a previous treatment has forced them to memorize. If they are not quickly brought up to the releasing site, they tend to pop up to their nominal diameter. It is therefore generally necessary to cool these endoprostheses and/or the applicator in which they are placed, as is also described in U.S. Pat. No. 5,037,427. In this document, the applicator of a memory-alloy stent is cooled throughout the placement phase by a ice-cooled physiologic saline. When the desired position is reached, the flow of cooling fluid is stopped and the stent, gradually warmed by the body heat, expands.
According to this method, it would be theoretically possible to remove the stent by cooling it again, so that it could be freely brought back to its reduced original diameter.
The truncated incised cylinders and the mesh structures generally lack flexibility, are rigid and kink excessively. There is thus a high risk that they damage the walls of the vessels. In addition, they take up considerable space in the introducer.
The helical structures (or coils) when triggered by the mere change of phase do not open the arteries sufficiently and are ineffective in the treatment of stenoses, since they do not cover the whole of the artery wall.
Furthermore, none of these endoprostheses types can be used in arteries of large calibre.
Moreover, it is necessary to anticipate the possibility that the stress force generated by the phase transition of the materials forming the framework is not sufficient to overcome the pressure due to the wall and the friction. In this case, there is a high risk of the endoprosthesis not being able to deploy.
The operator must anticipate the possibility of subsequent introduction of an inflatable balloon in order to bring the endoprosthesis to its nominal diameter. This technique of "forced" widening frequently leads, in the long term, to reactions by the organism (in particular tissue proliferation).